Second hand Opens are a bit like hens teeth - very rare...
Let us know how you get on with looking at your local club- My local one, which I am a member of has a stock of opens for Club use on club days, mainly Old Towns, but the club is mainly a kayak one so they tend to sit there.
People in the club who paddle opens tend to own their own ones. Prices vary for new from @£500 ish up to £1200 plus depending on what they are made from. Paddles vary from £25each up to over £100 again depending on what you want to spend and how hi-tech you want to go.
Another option is if you are a member of the Camping and Caravanning Club is to look at their special interest section, the Canoe Camping Club. They run a come and try for members of the main club in June each year at the Chertsey site and also I think in Yorkshire Aug/Sept time. People can try Opens or Kayaks as they wish, and there is quite a good training structure in the club to progress to 1-2*, at various camping events.
I am also a member of the Open Canoe Association, a group which specialises in, not surprisingly Open Boating - not an opportunity to "have a go" but there are quite a lot of trips, and once again training if you are a BCU or OCA member - for example, in Devon last weekend my son and I passed our 3* Open assesment with them.
Good hunting,
Graham